Method of removing hydrogen from molten metal



United States Patent IVIETHOD 0F REMOVING HYDROGEN FROM MOLTEN METAL Joseph Gordon Emmott, Birmingham, England, assignor,

hy mesne assignments, to Foundry Services International Limited, Birmingham, England, a British company No Drawing. Filed Dec. 16, 1957, Ser. No. 702,751

Claims priority, application Great Britain Feb. 4, 1957 6 Claims. (Cl. 75-93) This invention relates to the production of non-ferrous metals, e.g. copper, nickel and alloys of copper and of nickel.

In the production of these metals it is a well known difficulty that the molten metal may contain occluded gases which tend to give rise to the presence of minute bubbles or porosities in the solidified metal. It is known in the case of copper alloys to bubble nitrogen through the metal in order to de-gas it. However, the method is diflicult to carry out in practice and no simple method of degassing such metals is indeed known.

It has now been discovered, however, that molten copper and nickel and their alloys and other molten nonferrous metals may be efficiently and simply degassed by generating within the body of the molten metal certain types of gas which will act to flush the metal and entrain occluded gases.

According to the present invention a method of removing a dissolved gas from molten non-ferrous metal comprises introducing into the molten non-ferrous metal, preferably towards the base thereof, a solid compound which at the temperature of the molten non-ferrous metal will generate carbon monoxide and/or carbon dioxide, the generated gas being either free from the gas which is to be removed or having a partial pressure of that gas which is less than the equivalent internal pressure of the said dissolved gas in the said molten metal. Generally the gas to be removed will be hydrogen and the expression equivalent internal pressure is used to define the hydrogen pressure in the metal equivalent to that partial pressure of hydrogen above the metal which would be in equilibrium with the dissolved hydrogen.

The generation of carbon monoxide and carbon diox ide may be achieved by introducing into the molten nonferrous metal such materials as dolomite (magnesium carbonate-calcium carbonate) limestone, preferably coated with fluorspar, marble and other alkaline earth metal carbonates, e.g. barium carbonate. The foregoing materials may with advantage be used in association with a proportion of carbon such as to ensure as far as possible that any carbon dioxide generated is converted to carbon monoxide, thus increasing the net volume of gas. Moreover, since the foregoing carbonate materials react very rapidly it is usually desirable to associate them with a proportion of a refractory material, such as magnesia, e.g. by reducing the materials to powder or granular mixture and compacting or binding them to a readily handleable shape. A material of particular value comprises marble chips and powder bonded together with a silicate, e.g. a mixture varying from 75 parts chips and 25 parts powder to 50 parts chips and 50 parts powder, bonded together in the proportion 90 parts of the mixture with 10 parts of sodium silicate to 80 parts of the mixture with 20 parts of sodium silicate. A preferred composition consists of 83-85 parts of the mixture with -17 parts of sodium silicate, the mixture being 70% chips and 30% powder. The composition is preferably in a form presenting a large surface area, e.g. as a fluted cylindrical block.

It is important to avoid introducing with the additive into the molten metal any important quantity of moisture and accordingly the additive material employed should generally have a moisture content below 1.5% by weight and preferably below 0.15%. Such low moisture content can be achieved by heating the additive to a temperature as high as possible but not reaching the temperature at which the additive commences to generate carbon dioxide or carbon monoxide. In the case of marble the heat treatment may be at temperatures of 400 C. or more but not exceeding about 600 C. at which temperature marble begins to decompose.

The presence of moisture in the additive causes a hydrogen concentration in the gas bubbles generated. Hence in order for the gas bubbles to be effective in entraining hydrogen from the non-ferrous metal it is necessary, as stated above, that the partial pressure of hydrogen in the generated bubbles should be less than the equivalent internal pressure of hydrogen in the metal.

The quantity of solid material addedto the molten metal in accordance with the invention, and the time dur- The following mixture is tableted using sodium silicate as binding agent:

Parts by weight Dolomite 88 Carbon 12 Tablets weighing /2 to 1 lb. of the said mixture are employed per ton of molten copper or copper-based alloy, the tablets being plunged when the metal is at 1000- 1200" C. 4

An excellent degassing is thus obtained.

A similar result is obtained by using, instead of the said tableted mixture, silicate bonded marble obtained by mixing together parts by weight of marble chips and 30 parts of marble powder and bonding parts of that mixture with 15 parts of sodium silicate and stoving the product at 400 C. for 6 hours, whereby the moisture content of the product is reduced to 0.12% by weight.

It will be appreciated from the foregoing that to secure an optimum degassing of non-ferrous metal the additive must have as low a moisture content as possible since the less hydrogen there is in the generated bubbles of carbon dioxide and/or carbon monoxide, the greater will be the efiect of these bubbles in entraining hydrogen from the non-ferrous metal. Whilst the foregoing example uses marble as the gas-generating agent any other solid substance which at the temperature of the molten non-ferrous metal will generate carbon dioxide and/or carbon monoxide may be used. Marble is a material of choice because of its cheapness but any carbonate particularly of an alkaline earth metal, and particularly any natural carbonate may be used subject only to the overall consideration that if a natural carbonate is used it should not simultaneously introduce into the non-ferrous metal melt any unwanted constituents.

Whilst the invention has been particularly described with reference to the treatment of copper and its alloys it is to be understood that these are only by way of example and that the invention is applicable to the removal of hydrogen from any non-ferrous metal, e.g. nickel, cobalt, chromium, molybdenum, vanadium, tin, gold and silver and alloys of any of these with each otheror with other non-ferrous metals or semi-metals.

I claim as my invention:

1. A method for removing hydrogen from molten nonferrous metal which comprises introducing into the molten non-ferrous metal a solid alkaline earth metal carbonate having a moisture content below about 0.15% obtained by heating said carbonate to between about 400 C. and about 600 C. to achieve incipient decomposition thereof.

2. A method for removing hydrogen from molten nonferrous metal which comprises introducing into the molten non-ferrous metal marble having a moisture content below about 0.15% obtained by heating said marble to a temperature between about 400 C. and about 600 C. to achieve incipient decomposition thereof.

3. A method for removing hydrogen from molten nonferrous metal which comprises introducing into the molten non-ferrous metal a material consisting of marble fragments bonded together with a silicate, said material having a moisture content below about 0.15% obtained by heating said material to a temperature of between about 400 C. and about 600 C. to achieve incipient decomposition of said marble.

4. A method for removing hydrogen from molten nonferrous metal which comprises introducing into the molten non-ferrous metal a material comprising an alkaline earth metal carbonate and carbon, said material having a moisture content below about 0.15% obtained by heating said material to a temperature of between about 400 C. and about 600 C. to achieve incipient decomposition of said carbonate.

5. A method for removing hydrogen from molten nonferrous metal which comprises introducing into the molten non-ferrous metal a material comprising dolomite and carbon, said material having a moisture content below about 0.15% obtained by heating said material to a temperature of between about 400 C. and about 600 C. to achieve incipient decomposition of said carbonate.

6. A method for removing hydrogen from molten metal consisting essentially of copper which comprises introducing into the molten metal consisting essentially of copper a solid alkaline earth metal carbonate having a moisture content below about 1.5% obtained by heating said carbonate to a temperature of between about 400 C. and about 600 C. to achieve incipient decomposition of said carbonate.

References Cited in the file of this patent 

1. A METHOD FOR REMOVING HYDROGEN FROM MOLTEN NONFERROUS METAL WHICH COMPRISES INTRODUCING INTO THE MOLTEN NON-FERROUS METAL A SOLID ALKALINE EARTH METAL CARBONATE HAVING A MOISTURE CONTENT BELOW ABOUT 0.15% OBTAINED BY HEATING SAID CARBONATE TO BETWEEN ABOUT 400*C. AND ABOUT 600*C. TO ACHIEVE INCIPIENT DECOMPOSITION THEREOF. 